The present invention relates to a carrier recovery system for use in a mobile communication system, and more particularly to the carrier recovery for application to a digital phase demodulator for demodulating digital phase-modulated signals by coherent detection in a mobile communication system.
Some digital phase demodulators for demodulating digital phase-modulated signals use coherent detection while others use differential detection. A digital phase demodulator using coherent detection recovers the carrier phase from received signals, generates a reference carrier signal, and coherently detects the received signal with the reference carrier signal.
A phase lock loop (PLL), such as the Costas loop, is frequently used for carrier phase recovery (see F. M. Gardner, Phaselock Techniques, New York: John Wiley & Sons, 1979, pp. 217-225). In order to prevent the degradation of the bit error rate due to phase jitters at a low carrier to-noise power ratio (C/N), the loop noise bandwidth of the PLL should be kept sufficiently narrow, somewhere between 1/50 and 1/200 of the modulation rate.
A carrier phase recovery system employing a PLL, because of the long acquisition time the loop takes to enter into a stable state after the first received signal inputting, is unsuitable for a time-division multiple-access (TDMA) system for burst signal transmission or a mobile communication system in which signal interruptions frequently occur.
There are also known open loop systems by which received signals are nonlinearly processed to extract the carrier component and the phases of the extracted carrier components are averaged over time to recover the original carrier phase. These systems take no long acquisition time and, moreover, the aquisition time is constant irrespective of the input phase condition. Yet, they require a long enough phase averaging time, or in other words a narrow enough bandwidth for the filter to achieve phase averaging. The operation of such open loop carrier phase recovery systems is analyzed in detail by A. J. Viterbi et al. (A. J. Viterbi, A. M. Viterbi, "Nonlinear Estimation of PSK-Modulated Carrier Phase with Application to Burst Digital Transmission", IEEE Transactions on Information Theory, vol. IT-29, No. 5, pp. 543-551, July, 1983).
In a terrestrial mobile communication system or a satellite-based land-mobile communication system, multipath fading takes place with the motion of the mobile terminal as a consequence of multiple reflections from topographies or buildings. Since the amplitude phase distribution of signals affected by this fading can be approximated by the Rice model, which is the amplitude phase distribution of the synthetic signals of direct-path and multipath waves (M. Schwartz, W. R. Bennet, S. Stein, Communication Systems and Techniques, New York: McGraw-Hill, 1966, pp. 372-374), this multipath fading is known as the Rice fading.
The range of the spectrum due to this fading, i.e. the fading pitch, is determined by the frequency of the carrier used and the velocity of the mobile terminal. If, for instance, the carrier frequency is 1.5 GHz and the maximum speed of the mobile terminal is 120 km/h, the fading pitch will be about 200 Hz at the maximum. Meanwhile, supposing that speech signals are transmitted after being encoded into high efficiency codes and further into error-corrected codes and quarternary phase shift keying (QPSK) is used for modulation, the modulation rate will be, say, 3.2 kbaud. If, in this case, the bandwidth of the PLL loop, or that of the filter for phase averaging, in a carrier phase recovery system is selected between 1/50 and 1/200 of the modulation rate, the bandwidth will be 64 Hz to 16 Hz, considerably narrower than the maximum fading pitch. Accordingly, the recovered carrier phase will be unable to track the phase of the received carrier affected by fading. As a result, the fast phase fluctuation the received carrier was subjected to by the fading will become phase errors of the recovered carrier. Therefore, if the received signal is demodulated with reference to this recovered carrier phase, it results in serious bit error rate degradation.
On the other hand, digital phase demodulators using differential detection, which are known to be relatively suitable for use in a fading environment, not only are intrinsically inferior in the bit error rate to those using coherent detection by 2 to 3 dB but also cannot avoid more degradation of the bit error rate in an environment where so fast phase fluctuations are invited by fading that the phase varies even within a bit period. In the worst conceivable fading environment for the operation of such a system, where the direct-path carrier to-multipath power ratio (C/M) is 7 to 10 dB and the fading pitch is about 1/16 of the modulation rate as referred to above, there will be no substantial difference in the bit error rate between differential detection and coherent detection.
As described above, digital phase demodulators by the prior art have nothing to compensate for the relatively fast phase fluctuations which arise in the Rice fading environment and are too great to be ignored.